Slurry phase reactions, particularly those occurring in bubble columns are well-known and need not be described here. Literature references for such systems are plentiful, for example, see Farley et al, The Institute of Petroleum, vol. 50, No. 482, pp. 27-46, February (1984); H. Storch, N. Columbis, R. B. Anderson, "Fischer-Tropsch and Related Synthesis", Wiley, 1951, New York; and J. Falke, "Advances in Fischer-Tropsch Catalysis, "Verlag, 1977, Berlin", as well as European Patent Application #88309904.6, publication no. 0313375.
Slurry phase reactions are often preferred over fixed bed processes because of easier heat removal in exothermic reactions and ease of contacting the reactants with the catalyst. Nevertheless, heat removal problems, for example, are not completely eliminated by changing from fixed bed to slurry phase processes. Often slurry phase reactions conducted in bubble columns require internal configurations for aiding heat removal in exothermic reactions, for improving contact between the reactants and the catalysts, and for preventing slumping of the bed, that is, where the catalyst particles accumulate near or settle at the bottom of the bubble column and inhibit both heat removal and reactant contacting, and otherwise affect the reaction deleteriously.
Slurry phase reactions, particularly those conducted in bubble columns can be improved dramatically by conducting the reaction in the presence of an additional solid material. The additional solid aids the fluidization of the primary catalytic material.
Catalyst particles in a bubble column tend to settle to the bottom of the column because of the influence of gravity and because of low liquid through-put rates typical of many slurry phase reactions. This settling tendency is opposed by dispersion forces created by rising bubbles of gas injected at or near the bottom of the reactor. Bubble column reaction conditions generally attempt to balance these opposing forces so that neither slumping of the catalyst bed nor carrying of the catalyst out of the column occurs. The balancing of these opposing forces results in an exponential distribution of the catalyst with the solids concentration decreasing by a factor of about 2.7 every time the vertical position in the reactor column is increased by an amount equal to the decay length, D/U.sub.s, where D is the dispersion coefficient and U.sub.s is the settling velocity of the catalyst particles. The dispersion coefficient depends on the superficial gas velocity through the system and on the effective diameter of the reactor column. On the other hand, the settling velocity can be expressed as: EQU U.sub.s =U.sub.o (1-c).sup.n
where
U.sub.o =d.sub.p.sup.2 (.rho.s-.rho.)g/18.mu.
and wherein c is the volume fraction of solids in the slurry, d.sub.p is the particle diameter, .rho..sub.s is the density the solids, .rho. is the density of the slurry liquid, g is the gravitational constant, .mu. is the viscosity of the suspending liquid, and n is a constant ranging from 4-8. Hence, changing the solids concentration, for example, from 10% to 30% should decrease the settling velocity, and therefore, increase the decay length of the solids profile, by a factor of from 3 to 7.
Explained in other terms, a bubble column reactor--which for purposes of this invention is any reactor containing solids slurried in a liquid and into which gas (or another liquid) is injected or sparged into the column at or near the bottom of the column and gas bubbles rise in the column--allows a reaction to occur wherever there is catalyst. The dispersing force of the gas increases as the superficial gas velocity increases. Thus, as the superficial gas velocity increases from zero to any positive number, the catalyst instead of slumping to its minimum bed height will occupy an "expanded" bed height that depends on the gas velocity. Also, the expanded bed will have a concentration profile that depends on gas velocity, catalyst particle size, particle density and total loading of the catalyst. Additionally, if the fluidizing gas is a reactant, as in hydrocarbon synthesis reactions, less gas will be available higher in the column as the gas rises and reacts in the column.
The reaction taking place in the bubble column, whatever that reaction may be, will effectively and substantially take place within the expanded bed, i.e., where the catalyst is located. Now, by increasing the solids loading in the slurry, all other things being equal, the bed height will increase. Because the reaction, whatever it may be, takes place in the region of the expanded bed, the reaction zone is lengthened, also. The effects of increasing the reaction zone are manifold: more uniform reaction rate; more uniform heat release profile in the reactor (for exothermic reactions) or heat absorption (for endothermic reactions); less severe mass transfer limitations in the bottom portion of the reactor, thereby allowing better catalyst productivity, selectivity, and lower catalyst deactivation rates. Additionally, since heat release or absorption is more uniform, that is, the same amount of heat is released or absorbed, but over a longer distance resulting in a lower heat flux per unit volume, heat exchange surface areas can be distributed over a longer distance in the column thereby allowing reactor internals to be less expensive and easier to maintain. Increasing the solids loading has the additional effect of producing a lower volume fraction of gas per unit volume of reactor with the prospect for higher reaction rates per unit of reactor cross sectional area, and to increased thermal conductivity of the slurry; The latter effect further improving heat transfer.